Sound reproducing instrument



Jan. 23, 1934. F. A. MITCHELL 1,944,258

SOUND REPRODUC I NG INSTRUMENT Filed July 2, 1928 3 Sheets-Sheet l INVENTOR Eng. 8 FRANK ALLEN ["UTCHELL.

Jan. 23, 1934. F, A WTCHELL 1,944,258

SOUND REPRODUC ING INSTRUMENT Filed July 2Q 1928 5 Sheets-Sheet 2 &

Fig. 9 a6 V 2 *6 Fig. 13

INVENTOR FRANK. ALLEN M \TCH'ELL.

' Jan. 23, 1934,

F. A. MITCHELL 1,944,258

SOUND REPRODUCING INSTRUMENT Filed July 2, 1928 3 Sheets-Sheet 3 INVENTOR FRANK ALLEN MFFCHELJL Patented Jan. 23, 1934 UNITED STATES SOUND REPRODUCING INSTRUMENT Frank Allen Mitchell, London, to Columbia Phonograph England, assignor Company, Inc.,

Bridgeport, Cnn., a corporation of New York Application July 2, 1928, Serial No. 289,846, and in Great Britain July 11, 1927 3 Claims.

This invention relates to sound reproducing instruments of the type involving an electric pick-up device for reproducing the sounds by electrical as distinct from mechanical means and .51 has for one of its objects to devise improvements in the construction and arrangement of such reproducers whereby a substantially even response will be obtained throughout the essential musical frequencies.

Further objects of the invention will be apparent from the description given'hereafter.

The invention consists in an electrical reproducer or pick-up of the type having an armature mounted to pivot about an axis located within the space bounded by four or more pole extensions of one or more magnets and having a stylus holder characterized in that one end of the stylus holder extends nearer the axis of pivotation of the armature than do the ends of the 2'0; said. pole extensions.

The invention also consists in an electrical reproducer or pick-up of the type having an armature or like moving part and a stylus holder characterized in that the said holder is provided 251 with a Wedge shaped hole adapted to grip the reproducing stylus when playing a record.

The invention also consists in an electrical reproducer or pick-up of the'type having an armature or like moving part and a stylus holder '3'0i characterized in that the armature or an associated part is pivoted by means of a single or multiple ball bearing.

A further object is to provide a novel mounting for the pick-up, the mounting and pick-up cooperating in such a manner that the pick-up itself is pivotally mounted whereby it may be moved about its pivot to an inoperative position.

The invention further consists in other details and arrangements hereinafter described with reference to the accompanying drawings, in

which: i

Fig. 1 is an elevation of an electrical reproducing device in accordance with the invention showing the parts assembled, but with the cover plate removed;

Fig. 2 is a sectional elevation on the line 2-2 of Fig. 1;

Fig. 3 is an underneath plan of Fig. 1;

'Fig. 4 is a perspective view of the armature and stylus holder removed;

Fig. 5 is a perspective view of the induction coil removed;

Fig. 6 is a side elevational view of the armature tel and stylus holder; 1

Fig. 7 shows several modifications in the shape of the stylus gripping hole;

Fig. 8 is an electrical circuit showing a convenient means of connection between the reproducing device and the associated amplifier;

Fig. 9 is an elevation of a modification of the electrical reproducing device in accordance with the invention;

Fig. 10 is a sectional elevation on the line 10-10, Fig. 9;

Fig. 11 is a detail view of the armature;

Figs. 12 and 13 show a convenient method for mounting the electrical reproducer on the cabinet;

Fig. 14 is an elevation of a further modification of the electrical reproducing device in accordance with the invention;

Fig. 15 is a sectional elevation on the line 15-15, Fig.14; and

Figs. 16 and 17 show modifications of the mounting arrangements suitable for the above reproducing devices.

In carrying my invention into efiect in one convenient manner as illustrated in Figs. 1 to3 I provide a casing 20, having amagnet 21 mounted therein and provided with pole pieces of soft iron, silicon steel, or other suitable magnetic material, so positioned that the magnetic air gap is at its maximum in a direction at right angles to the plane of the magnet 21.

In the construction shown each of the pole pieces is formed with a pair of extensions 22 and 23 or 24 and 25 having the same polarity, (i. e. 22 and 23 may be north while 24 and 25 are south) the ends of the extensions overlapping one another so that between the extensions 22 and 24 or 23 and 25 there is a magnetic air gap in which the ends of the armature 26 may be located.

The armature 26 preferably consists of a light bar of steel, soft iron or the like, which is conveniently formed in one with the stylus holder 27 and which is pivotally mounted on the pole pieces so that the pivot axis 28 lies in the space bounded by all four of the pole piece extensions 24, 25, 22 and 23...

The pivoting of the armature 26 is accomplished in any suitable manner which will prevent the armature freezing on the pole extensions when moved out of its normal rest position, and in the example shown in Figs. 1 to 6 thev pivots. are formed by flat spring members 29 and 30, the planes of which are at right angles to or otherwise intersect one another and thus form an axis around which the armature 26 can pivot.

In order that the armature shall weigh as little as possible, the stylus holder 2'7 forms part of the armature 26 and is situated between the two lower pole pieces, so that one end of the stylus holder extends nearer the axis of pivotation 28 of the armature than do the pole pieces 22 and 24.

A portion of the armature 26 is arranged within a coil of wire 31 which is located within the space formed within or bounded by the pole piece extensions 24, and 22, 23, which coil functions as an inductive armature winding for generating electrical impulses when influenced by movement of the armature relative to the magnet pole pieces 32 and 33, and in order to enable a relatively large quantity of wire to occupy a minimum space, the

bobbin 34 on which the coil 31 is wound may be cut away on two opposite sides, preferably as shown in Fig. 5, this feature being also of advantage in reducing capacity effect between the component parts.

When the reproducer is in operation on 2. record, the stylus 35 (Fig. 6) is vibrated by the sound groove in the usual manner, thus causing the stylus holder 27 and armature 26 to vibrate about its axis of pivotation 28, so that the end 35 of the armature furthest from the stylus point is caused to approach one of the pole extensions 25 for example, while the lower stylus end 27 of the armature approaches the pole extension 22. When the armature is thus moved during one half cycle of a sound wave it will become momentarily magnetized so that it is north at its upper end and south at the stylus holder end, and thus there is created a magnetic field which passes through the coil of wire 31 surrounding the armature 26 so that an electric current is momentarily generated in such coil.

During the next half cycle of the sound wave impulse, the armature is moved in the opposite direction and its polarity is thus reversed, and thus again a momentary current is generated in the coil in the reverse direction to that previously produced. 1

It will be seen that vibrations of the armature 26 to and from the pole pieces 24, 23, or 22, 25 while playing a record, will generate an alternating electric current in-the coil 31 having a value dependent upon the amplitude of the sound wave vibration, and it will be clear that the electrical impulses thus produced may be amplified by means of a thermionic valve or other amplifying device and a loud-speaker or like sound reproducing apparatus, it being preferable that the electrical output from the reproducer be connected to an auto-transformer or the like by which means the intensity or volume of the reproduction can be regulated.

. One convenient form of regulator of the above type is shown diagrammatically in Fig. 8 by way of example. The coil 31 of the reproducer is connected through the conducting leads 36 and 37 to an auto-transformer 38 which may be in the form of a tapped choke or resistance.

This tapped choke 38 is in turn connected to the input transformer 39 of the amplifier through the regulating contact arm 48, by movement of Which the overall intensity of the alternating audio frequency electric current delivered by the reproducer coil 31 to the input transformer 39 can be varied as required, thereby regulating the volume of sound generated by the amplifier and its associated apparatus.

When it is required to alter the value of the intensity of lower audio frequencies relative to higher audio frequencies, any Well known filter circuit may be used, for example, it-may be convenient to provide shunting or blocking condensers 41 and 42 in the connecting circuit somewhat as shown in Fig. 8 so that it is possible to shunt the high frequencies by means of the condenser 41 or to block the low frequencies by means of the condenser 42, and such conditions as are required to give substantially even response throughout the essential musical frequencies to compensate for inaccuracies in the record or its associated apparatus may be regu-- lated by means of a contact device 43 for example.

In some constructions it is desirable to have means in conjunction with the above-mentioned spring pivot mounting 29, 39 to prevent excessive movement of the stylus bar, and we may therefore employ such means as that described in the specification of our prior British Patent No. 269,978, which may take the form of two rubber or like pads 44 and 45 (Fig. 2) adapted to gently press each side. of the armature extension 46 and maintain same in an approximately central position between the pole pieces 23 and 25. The pads 44 and 45 may be adjusted by means of a screw, screws or the like operable from the exterior of the case 20 as shown at 47, 48 (Fig. 2) although in some cases, however, only one resilient member 44 or 45 need be adjustable.

As already stated, it is important that the weight of the armature and stylus holder shall be as small as possible without undue restriction of the effective mass, so as to permit the armature to vibrate at all musical frequencies: 1' may therefore construct the stylus holding portion of the armature in such a manner that the needle or stylus 49 is held rigid in the holder 2'? while playing a record without the necessity for using the usual screw grip. This maybe accomplished for example by forming the needle hole 50 in the stylus holder 27 so that it is diamond shaped, or the wedge shaped hole may be of any suitable form as for example, those shown inI-ig. '7, the arrangement being such that when a needle or stylus 49 is placed in the holder 27 and pulled to one side thereof by the drag of the'needle 49 on the record 51 (Fig. 12) which is being reproduced, the needle will be tilted as shown in Fig. 6 and jammed into the two opposite acute angle corners of this diamond or wedge shaped hole 50, and will thereby transmit all lateral vibrations of the needle to the stylus holder and armature 26. V

If necessary or desirable, a small spring 52 may be adopted for preventing the needle or stylus falling out when not reproducing a record, or a lever or other suitable means may be adopted; for example, the needle may be retained by the magnetic attraction of the pole pieces 22,24 or an associated part, When required, the top or inner end 53 of the needle hole may be inclined to guide the needle or stylus into its correct position when placed on a record and the top or inner end of the hole may also contain a rubber or resilient pad to damp unwanted vibrations in the needle.

A stylus holder having a wedge shaped hole to retain the needle or stylus is shown in the two reproducing units illustrated in Figs. 1 to 13 but itmust be understood that such stylus holding means maybe equally well applied to any other form of electrical reproducing device or pickup especially those in which an armature is used.

In the construction shown in Figs. .9 and 11, I also make use of a combinedstylus holder and armature which operates somewhat in the same manner as that shown in Figs. 1 to 3.

To obtain an even lighter armature than that shown in Fig. 4, I shorten the upper limb of 26 as shown in Fig. 11, and position it between the pole extensions 22, 23 and 24, 25 so that the end 54 occupies a position close to but not between the upper pole extensions 23, 25. In such a position it is able to approach either of the upper poles'without ever touching or freezing on to them;

The armature in this construction is mounted upon ball bearings, comprising one ball 55, 56 on each side of the armature pivot 57, and to maintain the armature in a normally central position rubber or like resilient-pads 47, 48 are provided between the lower pole extensions'22, 24. The armature is encircled-by an induction coil as in the previous example, and in the example illustrated, one conducting lead 58 is attached direct to the metal of the case 20 by means of the screw 59 or by any other means, while the other conducting lead 60 passes out of the ,case as shown.

The purpose of this arrangement will be observed in connection with Figs. 12 and 13 which show the reproducing unit 20 mounted on the pivots 61 in the forked member 62, which is in turn mounted on the reproducer-supporting movable arm 63 pivoted on the cabinet 64 of the instrument at 65.

One of the before mentioned conducting leads 60 is connected to the arm 63 and thence to the lead 36 which may be connected to the amplifier as shown in Fig. 8 for example, or in any other known manner. The case 20 which as already stated, is connected to lead 58 is in direct metallic contact with the lead 37 which is also connected to the amplifier.

So that there shall be no electrical connection between the fork 62 and the arm 63, the fork is mounted upon a rubber tube 66 and is held in place by the clamping screw 67 pressing on the insulating washer 68. This rubber tube or any other resilient mounting used in its place also serves to damp out any mechanical resonance in the reproducer support which may react on the stylus and armature of the reproducing unit.

When the reproducing unit is in operation on a record disc 51 and is in the position shown at 20 in Fig. 12, the electrical impulses are conveyed along the two metallic paths 20, 62, 37 and 60, 63, 36 to the amplifier, but when the reproducing unit is thrown back into the rest position 69, it touches the screw head 67 and thereby short circuits the leads 36 and 37 so that the reproducing unit is virtually disconnected from the amplifier.

While the above switching arrangement is shown by way of example, it must be understood that other switching arrangements may be used for similar purposes.

When it is desired to obtain maximum efficiency from the reproducing device, it may be desirable to mount the reproducing device so that it is supported upon two or more link members or arms adapted to vary the angle of the reproducing device in relation to its support while playing a record.

One convenient arrangement to obtain the above result is shown in Fig. 16 in which the reproducing unit 70, which may be of the form illustrated in Figs. 14 and 15, is mounted upon pivots 71 between the fork 72 which is in turn mounted upon the reproducer supporting links or rods 73 and 74, which are pivoted upon the cabinet of the instrument at 75 so that they operate after the manner of a parallel ruler to enable the stylus 49 to maintain a more perfect track alignment relative to the record groove on the disc 57 while playing a record, which, as will be seen in the illustration, is adapted to maintain the stylus 49 in a vertical plane which is approximately tangential to the sound groove, throughout its movement from the position, 76 to the dotted position 77.

Fig. 17 shows another convenient arrangement for obtaining a similar effect, in which three links 78, 79 and 80 are used, and having two of the. links 78 and 79 enclosed within the third hollow link 80. The links are attached to a forked member 81 which is adapted tocarry a reproducing device 20 which may be of the type disclosed in Figs. 1 to 3, there being a pivot connection at 82 between the fork and case 20.

When required the links 73 and 74 or 78 and 79 may also function as electrical conducting leads connecting the reproducer to the amplifier, and in Fig. 16 the leads 83 and 84 (Fig. 14) are shown connected direct to the links 73 and 7,4. It is obvious that switching arrangements of the type disclosed with reference of Figs. 12 and 13 may be used with the above multiple-link reproducer supports if required, and. that the reproducer supports may be combined with any known auto-stopping device to stop the rotation of the record when the reproduction has concluded.

It will have been noted that the reproducing unit 70 shown in Fig. 16 is arranged horizontally relative to the record instead of vertically as in the types shown in Figs. 1 and 9. Such change in the plane of operation can conveniently be arranged as shown in Figs. 14 and 15 in which the armature 26 and stylus holder 85 are mounted to pivot about an axis 28 locatedwithin the space bounded by the four pole extensions 22, 23, 24 and 25 but is adapted to rock in a'direction at right angles to that shown in Figs. 1 and 9.

The pivot 28 is mounted in multiple ball bearings 86 and 8-7, but otherwise operates in exactly the same manner as the armature in Fig. 9, or alternatively, I may employ an ordinary knife edge or other suitable form of pivot. The stylus holder 85 has the stylus held in place by the usual screw grip instead of the V-shaped hole as is used in the previous examples, and the armature 26 and stylus holder 85 (which, as before, forms part of the armature) is centralized between the pole pieces 22 and 24 by the rubber or like pads 47 and 48 which may be adjusted by screws or the like 88 or 89, or we may obtain stabilization by means of springs, a diaphragm, or other suitable connection.

It is obvious that any of the above-mentioned components may be used in duplicate, and in any form of reproducer in accordance with the invention I may employ two magnets, one pole of each of which is provided with two extensions while the other pole of each magnet has a single extension situated between the double pole of its opposing magnet and a pair of armatures is attached to a single stylus holder which may or may not be of magnetizable material and is pivoted between the six pole extensions of the magnets in such a manner that the ends of the armatures are situated in the magnetic air gaps between the pole extensions in a similar manner to that before described.

Vibrations of the armature while reproducing 2.

record will cause electric'current impulses to be generated in one or more coils surrounding the armature as already described, but it will be clear that many other modifications of the invention can be devised without departingfrom the scope of the invention.

It is to be understood that the invention is not to be limited to the foregoing details of con-' struction which are given by way of illustration only and component parts may be multiplied, modified, or equivalent arrangements. used to suit the particular purpose for which the device is to be employed or any practical requirements that may have to be fulfilled while at the same time the novel features of construction hereinbefore described may be 'reversely operated or combined with other and known components such for example as a reproducing device as in copending application No.'21,687/27 without departing from the scope of the invention.

Having now particularly described and ascertained the nature of the said invention and in what manner the same is to be performed, I declare that what I claim is:-

1. In an electrical reproducing device, a magnet, four pole extensions on the magnet, a pivotally mounted armature, the pivot for said armature located within the space bounded bythe four pole extensions, said armature including a stylus holder, and the stylus holder portion of the armature being located in a magnetic air gap between the pole extensions, and the other end of the armature being located. close to, but not in, a second air gap between pole extensions.

2. In an electrical reproducing device, a field magnet having four pole extensions, a pivoted armature, the pivotal mounting for the armature located within the space bounded by the pole extensions, said pole extensions arranged to extend in the plane of the field magnet, the axis of the armature pivot being arranged approximately in the plane of the field magnet, and the said pole extensions over-lapping one another at their extremities.

3. In an electrical reproducing device, a field magnet having four pole extensions, a pivotally mounted armature, the pivot for said armature being located within the space bounded by the pole extensions, said pole extensions arranged to extend in the plane of the field magnet, and said pole extensions over-lapping one another at their extremities.

FRANK ALLEN MITCHELL. 

